A handful of compact objects with a high mass companion star emit most of their energy above 0.1 GeV. Models involving an accreting black hole launching a relativistic jet (a microquasar) or a young pulsar have been proposed. The detection of radio pulsations securely proved the latter hypothesis only in a few systems. The case of LS I +61° 303 turned out to be a very difficult one. The varied phenomenology of this system includes changes in the morphology of the radio emission at different orbital phases and a couple of energetic, short magnetar-like bursts. The detection of transient radio pulsations with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) eventually identified a 0.269 s rotating neutron star in the system. A model based on a young (~5-10 kyr) magnetised (>5×10^12 G) neutron star alternating between rotation-powered and propeller emission along the eccentric orbit best explains the properties of the system. According to such a scenario, LS I +61 303 would likely host a magnetar in a binary system, closely related to the assumed progenitors of high-mass X-ray binaries (so-called ante-diluvian pulsars).
Radio pulsations from a neutron star within the gamma-ray binary LS I +61 303